ABSTRACT The burden of pediatric mental, neurological, and substance (MNS) disorders in low-and middle-income countries (LMICs) is tremendous. Although efforts have been made to improve mental health interventions and services for young children, and a number of evidence-based interventions (EBIs) developed in high-income countries have been successfully transported and demonstrated to be efficacious for improving children?s mental health in LMICs, many critical gaps remain. From the effectiveness research perspective, most EBIs adapted to LMICs have not been scaled widely, examined effectiveness and mechanisms, or focused on preventive interventions in early childhood. From the implementation perspective, most EBIs in LMICs rely on task-shifting approach of implementation because of shortage of mental health professionals (MHPs); however, challenges related to task-shifting have rarely been addressed. The overall goal of this application is to respond to these gaps by studying effectiveness of a population-approach EBI to promote early childhood mental health in one LMIC-Uganda, as well as simultaneously studying strategies that address task-shifting challenges to maximize EBI effectiveness. The EBI considered in this study is ParentCorps-Professional Development (PD), a school-based EBI that trains and support teachers to apply EBI strategies to promote child mental health. The PD approach represents a task-shifting model of mental healthcare by shifting preventive duties from professionals to teachers to promote children?s mental health. Two efficacy studies conducted in Uganda and Nepal have demonstrated feasibility of task-shifting, and positive impacts on children, teachers and schools The proposed study builds on the positive evidence and further study effectiveness, cost-effectiveness, and underlying mechanisms of PD using a scalable implementation approach in multiple regions in Uganda. In addition, based on previous findings that nearly 80% of Ugandan teachers (the EBI strategy users) experience occupational stress which threatens PD uptake, effectiveness, and sustainment, we propose to test a teacher stress management package (T-Wellness) as an enhancement to PD. Using a hybrid cluster randomized controlled trial (cRCT), we will randomize 36 schools with 540 teachers and 1,980 parent-child pairs to one of three conditions: PD + T-Wellness (PDT), PD alone (PD), and control. The Specific Aims are: 1) to evaluate the short- and longer-term effectiveness of PD and PDT, relative to control; 2) to examine effectiveness mechanisms and theory of change underlying the PD and PDT; and 3) to examine implementation contextual factors and mechanisms that contribute to effective task-shifting and teachers? uptake of EBI strategies within PD and PDT schools.